The present disclosure relates to semiconductor light-emitting devices each including a nitride-based semiconductor stacked structure including an active layer which has a nonpolar plane or a semipolar plane as a principal plane and emits polarized light, and methods for forming recesses of the same. The present disclosure also relates to light source apparatuses using the semiconductor light-emitting devices.
Nitride semiconductors containing nitrogen (N) as a group V element have been expected as a material of a short wavelength light-emitting device because of their band gap size. Gallium nitride-based compound semiconductors, in particular, have been actively researched, and blue light-emitting diodes (LEDs), green LEDs, and blue semiconductor laser diodes that use a gallium nitride-based compound semiconductor have been also commercialized.
Gallium nitride-based compound semiconductors include a compound semiconductor obtained by substituting at least one of aluminum (Al) or indium (In) for part of gallium (Ga). Such a nitride semiconductor is represented by the general formula AlxGayInzN (where 0≦x<1, 0<y≦1, 0≦z<1, and x+y+z=1). The gallium nitride-based compound semiconductors are hereinafter referred to as GaN-based semiconductors.
The replacement of Ga atoms with Al atoms in a GaN-based semiconductor allows the band gap of the GaN-based semiconductor to be wider than that of GaN, and the replacement of Ga atoms with In atoms in a GaN-based semiconductor allows the band gap of the GaN-based semiconductor to be narrower than that of GaN. Thus, not only short wavelength light, such as blue or green light, but also long wavelength light, such as orange or red light, can be emitted. From such a feature, nitride semiconductor light-emitting devices have been expected to be used for, e.g., image display devices and lighting devices.
Nitride semiconductors have a wurtzite crystal structure. In FIGS. 1A, 1B, and 1C, the plane orientations of the wurtzite crystal structure are expressed in four-index notation (hexagonal indices). In four-index notation, crystal planes and the orientations of the planes are expressed using primitive vectors expressed as a1, a2, a3, and c. The primitive vector c extends in a [0001] direction, and an axis in this direction is referred to as a “c-axis.” A plane perpendicular to the c-axis is referred to as a “c-plane” or a “(0001) plane.” FIG. 1A illustrates, not only the c-plane, but also an a-plane (=(11−20) plane) and an m-plane (=(1−100) plane). FIG. 1B illustrates an r-plane (=(1−102) plane), and FIG. 1C illustrates a (11−22) plane. Herein, the symbol “−” attached to the left of one of parenthesized numbers indicating the Miller indices expediently indicates inversion of the number, and corresponds to each of “bars” in some of the drawings.
FIG. 2A illustrates a crystal structure of a GaN-based semiconductor using a ball-and-stick model. FIG. 2B is a ball-and-stick model obtained by observing atomic arrangement in the vicinity of the m-plane surface from an a-axis direction. The m-plane is perpendicular to the plane of the paper of FIG. 2B. FIG. 2C is a ball-and-stick model obtained by observing atomic arrangement of a +c-plane surface from an m-axis direction. The c-plane is perpendicular to the plane of the paper of FIG. 2C. As seen from FIGS. 2A and 2B, N atoms and Ga atoms are located on a plane parallel to the m-plane. On the other hand, as seen from FIGS. 2A and 2C, a layer in which only Ga atoms are located, and a layer in which only N atoms are located are formed on the c-plane.
Conventionally, when a semiconductor device is to be fabricated using a GaN-based semiconductor, a c-plane substrate, i.e., a substrate having a (0001) plane as its principal plane, has been used as a substrate on which a nitride semiconductor crystal is grown. In this case, spontaneous electrical polarization is formed in the nitride semiconductor along the c-axis due to the arrangements of Ga and N atoms. Thus, the “c-plane” is referred to as a “polar plane.” As a result of the electrical polarization, a piezoelectric field is generated along the c-axis in an InGaN quantum well layer forming a portion of a light-emitting layer of a nitride semiconductor light-emitting device. Due to the generated piezoelectric field, the distributed electrons and holes in the light-emitting layer are displaced, and the internal quantum efficiency of the light-emitting layer is decreased due to a quantum-confined Stark effect of carriers. In order to reduce the decrease in the internal quantum efficiency of the light-emitting layer, the light-emitting layer formed on the (0001) plane is designed to have a thickness of not more than 3 nm.
Furthermore, in recent years, consideration has been made to fabricate a light-emitting device using a substrate having an m- or a-plane called a nonpolar plane, or a −r- or (11−22) plane called a semipolar plane as its principal plane. As illustrated in FIG. 1A, m-planes of the wurtzite crystal structure are parallel to the c-axis, and are six equivalent planes orthogonal to the c-plane. For example, in FIG. 1A, a (1−100) plane perpendicular to a [1−100] direction corresponds to one of the m-planes. The other m-planes equivalent to the (1−100) plane include a (−1010) plane, a (10−10) plane, a (−1100) plane, a (01−10) plane, and a (0−110) plane.
As illustrated in FIGS. 2A and 2B, Ga and N atoms on the m-planes are present on the same atomic plane, and thus, electrical polarization is not induced in directions perpendicular to the m-planes. Therefore, when a light-emitting device is fabricated using a semiconductor stacked structure having an m-plane as its growth surface, a piezoelectric field is not generated in a light-emitting layer, and the problem where the internal quantum efficiency is decreased due to the quantum-confined Stark effect of carriers can be solved. This applies also to the a-plane that is a nonpolar plane except the m-planes, and furthermore, even when the −r-plane, the (11−22) plane, a (20−21) plane, or a (20−2−1) plane called the semipolar plane is used as the growth surface, instead of the m-plane, similar advantages can be provided.
A nitride-based semiconductor light-emitting device including an active layer having a nonpolar- or semipolar plane as a growth surface (principal plane) has polarization characteristics resulting from the structure of the valence band thereof.
Japanese Unexamined Patent Publication No. 2008-109098 describes a light-emitting diode device aiming at reducing variations in the intensity of light due to the variations among the in-plane azimuth angles of light in a chip-arrangement surface. The light-emitting diode device includes light-emitting diode chips each including a light-emitting layer having a principal plane, and a package having a chip-arrangement surface on which the light-emitting diode chips are arranged. The light-emitting diode device has a configuration in which light emitted from the principal plane of the light-emitting layer has a plurality of different intensities of the light depending on the in-plane azimuth angles in the principal plane of the light-emitting layer, and at least either of the light-emitting diode chips or the package reduce variations in the intensity of light exiting from the package due to the variations among the in-plane azimuth angles of the light in the chip-arrangement surface.
Japanese Unexamined Patent Publication No. 2010-074008 describes a semiconductor light-emitting device aiming at obtaining high light extraction efficiency at which light is extracted from a surface of a light extraction side of the semiconductor light-emitting device and a good light distribution. The semiconductor light-emitting device includes a plurality of concaves provided on a light extraction surface of a semiconductor stack opposite to a surface to be mounted on a substrate, the semiconductor stack including a light-emitting layer between an n-type semiconductor layer and a p-type semiconductor layer. Each concave has two slopes having different slope angles in a direction in which the diameter of the concave is reduced from an opening of the concave toward a bottom of the concave. One of the slopes which has a gentle slope angle is a slope provided with irregularities, and the other of the slopes which has a steep slope angle is a flat surface.
Japanese Unexamined Patent Publication No. 2008-305971 describes a light-emitting device aiming at limiting the reduction in power efficiency of polarized light generated in an active layer. The light-emitting device includes a light-emitting section and an output section. The light-emitting section is made of a group III nitride semiconductor including a nonpolar plane or a semipolar plane as a principal plane, and includes a first semiconductor layer of a first conductivity type, an active layer, and a second semiconductor layer of a second conductivity type stacked in this order to emit polarized light from the active layer. In the output section, a plurality of stripe-like grooves extending in a direction vertical to the polarization direction of the polarized light are arranged in the polarization direction, so that the output section serves as an output surface having a sawtooth waveform. Light from the light-emitting section is transmitted through the output section, so that polarized light is output from the output surface.
Japanese Unexamined Patent Publication No. 2010-177455 aims at providing a long-life and highly reliable nitride semiconductor device including a substrate back electrode exhibiting good ohmic contact performance, high adhesiveness, and high heat resistance while the flatness of a back surface of a nitride semiconductor substrate is maintained. This nitride semiconductor device includes the nitride semiconductor substrate including a first surface and a second surface facing each other, an device structure provided on the first surface, and an electrode provided on the second surface. Grooves provided with irregularities on bottoms of the grooves and a nitrogen polar flat portion are provided on the second surface. The electrode is provided to cover the grooves.